Pulse generator



April 1966 J. A. HULL 3,244,911

PULSE GENERATOR Original Filed Oct. 31. 1957 7 Sheets-$heet l JOSEPH A. HULL INVENTOR.

BYwwA ATTORNEY April 5, 1966 J. A. HULL 3,244,911

PULSE GENERATOR Original Filed Oct. 31, 195'? 7 Sheets-Sheet 2 JOSEPH A.HULL

INVENTOR.

ATTORNEY April 5, 1966 J. A. HULL 3,244,911

PULSE GENERATOR Original Filed Oct. 51, 1957 '7 Sheets-Sheet 5 JEBT" JOSEPH AHULL INVENTOR.

$ BY W1W ATTORNEY April'5, 1966 J HULL 3,244,911

PULSE GENERATOR Original Filed Oct. 31, 195'? '7 Sheets-Sheet 4 JOSEPH A. HULL INVENTOR.

ATTORNEY April 5, 1966 i J. A. HULL 3,244,911

PULSE GENERATOR Original Filed Oct. 31, 1957 '7 Sheets-Sheet 5 JOSEPH A. HULL INVENTOR.

BY YYL W ATTORNEY A ril 5, 1966 J. A. HULL 3,244,911

PULSE GENERATOR Original Filed Oct. 31, 1957 '7 Sheets-Sheet 6 PERCENT TRANSMlSSION 20 PERCENT OF FULL OPEN VOLTAGE APPLIED TO KERR CELL JOSEPH A.HULL

INVENTOR.

BY WM; Y L W ATTORNEY April 5, 1966 J. A. HULL 3,244,911

PULSE GENERATOR Original Filed Oct. 51, 1957 7 Sheets-Sheet 7 JOSEPH A. HULL INVENTOR.

ATTORNEY United States Patent 3,244,911 PULSE GENERATOR Joseph A. Hull, Danvers, Mass, assignor, by mesne assignments, to Unilectron, Inc., Cambridge, Mass, a corporation of Massachusetts Original application Oct. 31, 1957, Ser. No. 693,570, now Patent No. 3,041,936. Divided and this application Apr. 3, 1962, Ser. No. 193,032

3 Claims. (Cl. 307108) This application is a division of application Serial No. 693,570, filed October 31, 1957, now Patent No. 3,041,936.

This invention relates to electronic signal generators and more particularly to an improved pulse generator especially useful in apparatus for taking pictures of objects moving at high speed, as as models of missiles under test.

The invention has particular application to studies wherein re-entry conditions of intercontinental ballistic missiles are aerodynamically simulated. It is customary for such simulation studies to be made in conjunction with a model of the missel which is projected at high speed in a ballistic range or tested in high speed wind tunnels. As techniques for obtaining high velocities have been improved, the demands on instrumentation required for measuring associated parameters have also increased. High speed photographic techniques used to record the position of models in flight, along with a history of the flow pattern surrounding the model, are particularly useful. If one considers a projectile travelling at a velocity of 20,000 feet per second, being photographed on film having a resolution of 20 lines per mm. at a magnification factor of /2, it will be apparent that it is necessary to limit the exposure time of the film to .01 microsecond in order to prevent image blur caused by motion of the projectile.

Briefly, the present invention comprises an improved pulse generator which is capable of delivering to a Kerr cell extremely large, high speed voltage pulses for operating the cell as a high speed optical light valve or shutter. The generator produces the pulse for the Kerr cell by discharging its energy through a spark gap which may also be arranged to produce radiant energy for illuminating the subject being photographed.

Through the improvements made in the pulse generator, it is possible to use a Kerr cell of unusually large proportions suitable for photographing high velocity subjects with a relatively large magnification factor and to assure automatic and perfect synchronization of the illumination of the subject With the operation of the Kerr cell.

In view of the foregoing it will be under-stood that it is a general object of the invention to provide improved means for taking high speed photographs, particularly photographs of subjects moving at extremely high velocities.

A more particular object of the invention is the provision of an improved pulse generator for delivering an extremely large voltage pulse of short duration to a Kerr cell. Still more specifically, it is an object to provide a transmission line pulse generator having pairs of parallel lines which are interconnected to provide voltage pulses of extremely fast rise time closely approaching square wave forms.

' The novel features that are considered characteristic of the invention are set forth in the appended claims; the invention itself, however, both as to its organization and methodof operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in conjunction with the accompanying drawings, in which:

FIGURE 1 is a perspective View of *a complete installation of a Kerr cell, pulse generator and associated components arranged for taking a shadowgraph picture of a high velocity projectile in a ballistic test range;

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FIGURE 2 is an exploded view of a Kerr cell showing its internal construction;

FIGURE 3 is a diagram of a trigger pulse circuit in association with a pulse generator and spark discharge device;

FIGURE 4 is a diagram of a multiple trigger pulse circuit in association with a pulse generator;

FIGURE 5 is a circuit diagram of a multiple trigger pulse circuit with delaying means for doubling the number of output pulses from an associated pulse generator;

FIGURE 6 is a diagram of the timing circuit used to provide the timed trigger pulses for circuits of the type shown in FIGURES 4 and 5;

FIGURE 7 is a graph showing the relationship between voltage applied to the Kerr cell and the transmission of light by the cell;

FIGURE 8 is a shadowgraph picture of a spherical projectile taken by means of the present invent-ion;

FIGURE 9 is a Lissajou diagram of elliptically polarized light Waves from a Kerr cell;

FIGURE 10 is a Lissajou diagram of plane polarized light waves from a Ker-r cell; and

FIGURE 11 is a schematic representation of a pulse generator in circuit with 8. Kerr cell.

KERR CELL PRINCIPLES Since 1875, when Kerr first demonstrated the electrooptical effect, Kell cells have been used as optical light valves or shutters. This type of shutter operates on the principle that certain normally isotropic substances such as Water, nitrobenzene, and carbon disulphide, become optically anisotropic when subjected to electrostatic stress by application of a potential difference to suitably arranged electrodes. The electro-optical properties of the Kerr cell are due to what is known as the Kerr eifect, which is the eifect of an electrical potential upon a substance having individual molecules Which possess aelotropic optical polarizability with reference to a set of axes within the molecule or a permanent dipole moment, or both. Upon application of a strong electric field to such a substance, the molecules assume a definite orientation due to the permanent dipole moment. The substantially regular arrangement of molecules causes the s-ubstance'to exhibit an over-all molecular asymmetry and optical anisotropy. Although electrical polarization alone will cause optical anisotropy, molecular orientation accounts for most of the Kerr effect in substances having a high Kerr constant.

With regard to optics, the significance of the Kerr effect is that a medium exhibiting such properties will, when subjected to "a strong electrical field, become doubly refracting or birefringent. Materials possessed of this property transmit radiant energy, such as light energy, at different speeds, depending upon the plane of vibration of the energy relative to the field. In other words, the materials are anisotropic when electrostatically stressed, having different properties in different directions. Thus, an anisotropic medium has indices of refraction which depend upon the plane of polarization and direction of propagation of the light waves passing through the medium.

The typical Kerr cell incorporates a pair of spaced plates, which may be electrically charged in a medium which exhibits the Kerr effect. Assuming that N is the index of refraction for light waves travelling perpendicular to the applied field with the plane of polarization parallel to the applied field and N, is the index of refraction for light waves travelling perpendicular to the field with the plane of polarization perpendicular to the field, Kerr established the following relationship:

ala

3 where N and N are the above defined indices of refraction, B equals the Kerr constant, B is the applied field in esu., and A is the wave length of the incident light in a vacuum.

If it is assumed that plane polarized light enters such an anisotropic medium with its direction of propagation perpendicular to the field and with its electric vector oriented at some angle tWllJh respect to the applied field, the light wave may be resolved into ordinary and extraordinary components parallel to and perpendicular to the direction of the field. The diiference in velocity of propagation of the parallel components and that of the perpendicular component will result in a relative phase shift of the components as the wave passes through the length of the field. The phase shift (d) is a linear function of the length of the field and may be calculated from the formula d=21rBLE where L is the length of propagation in centimeters, B is the Kerr constant, and E is the applied field in esu. This phase shift causes the emergent light wave from the Kerr cell to be elliptically polarized. For a relative phase shift d=1r radians, a special case results in which the emergent wave is plane polarized.

FIGURE 9 illustrates elliptical polarization. In this figure, X represents the extraordinary component which is phase shifted by approximately 1r/ 4 radians relative to the ordinary wave O.' This figure, constructed by Lissajous mechanics, illustrates that these waves combine to form an e'llip tic'a'lly polarized emergent wave. If voltages proportional to the component waves are impressed on the horizontal and vertical deflection plates of a cathode ray tube, an elliptical pattern such as P results on the face of the tube.

In FIGURE 10, X represents the extraordinary component which is phase shifted by 11' radians relative to the ordinary component, 0 resulting in plane polarization of :the emergent wave from the Kerr cell. This is indicated by the linear trace P determined by Lissajous inec'hanics. A similar trace would appearon the face of a cathode ray tube wherein deflection of the electron beam is proportional to the component waves.

From "the foregoing it will be apparent that if a polarizer is oriented so that the light entering a Kerr cell is plane polarized with its electric vector at an angle of -=45 with the applied field across the Kerr cell, and an analyzer is positioned after the Kerr cell with its plane of polarization oriented at 90 with respect to that of the polarizer, there must exist a combination of length and applied voltage for the Kerr cell which will produce a phase shift d=1r radians. Such phase shift results in the emergent beam being plane polarized in the plane of the analyzer as can be readily shown by Lissajous mechanics. For a given value of L, the necessary potential V in volts may be readily determined from the foregoing equation, bearing in mind that the field strength is proportional to the gradient of the voltage across the plates. Thus:

V 2 d 1r 21rBLE 21rBL V=300D 2 B where D=tl1e distance between the plates in centimeters.

During the time the field is applied, the effective rotation of the plane polarized wave by the Kerr cell permits light to pass from the polarizer, through the Kerr cell and the analyzer. Upon interruption of the field, the Kerr cell becomes optically isotropic and no light will pass through the analyzer, since it is crossed relative to the polarizer. Hence, such an arrangement may be used as an optical shutter.

A consideration of the foregoing equations will make it apparent that the voltage must increase linearly as the distance D between the plate increases. Because of the unavailability of high potential sources, Kerr cells in the past had very small apertures or narrow spacing between the plates and were therefore of limited utility. By applying a very large voltage to the plates, it is possible to provide spacing equivalent to a large aperture, or 1 number in photographic terms. This obviously increases the utility of the Kerr cell for extremely high speed photography. With wide plate spacings, it is possible to use a long focal length lens capable of producing a large image at the film plane. In this way the poor resolution of fast film-s can be offset somewhat.

Wide plate spacing dictates that the voltage must be quite high or the length of the cell must be made quite long. Lengthening the cell increases absorption of light rays within the cell and adversely affects the angle of View of the light system. It is therefore desirable to keep the length as short as possible, consistent with reasonable values of applied voltage.

Nitrobenzene is widely used in Kerr cells because of its relatively high Ker-r constant (346.0 10 esu.). As will be evident from the last equation, this also minimizes the amount of voltage necessary to produce the necessary phase shift within the cell. Electrical conductance through nitrobenzene is sufiiciently small that it may be neglected and the Kerr cell appears as a pure capacitive load on the circuit used to drive it.

Assuming that it is desirable to use a camera lens hav ing a focal length of 5" with an aperture of f/ 3.5, separation of 3.5 centimeters would be required for an assumed plate length equal to 10.5 centimeters. The capacitance C of the plates can be determined from the equation where C is the capacitance in uni, K is the dielectric constant between the plates (about 22 tor nitrobenzene at frequencies above 10 /isecond), A is the area of one plate in om and b is the distance between the plates in centimeters. For the assumed values, the capacitance of the plates will be 28 rf. and the required voltage to produce the desired phase shift will be approximately 40 kil'ovolts.

ShOlWl'l in FIGURE 11 is an equivalent circuit for the Kerr cell K and a pulse generator G for driving the cell. If the output of the generator is a square-Wave voltage pulse of duration 5 l0 seconds, a suitable internal resistance R of the generator may be defined as that resistance which, when combined with the capacitance of the Kerr cell, will produce a charging or discharging time constant equal to one-tenth of the pulse duration. For the Kerr cell whose proportions have been assumed above, the time constant T=RC=.5 X 10- Substituting the value of the Kerr cell capacitance, the in ternal impedance or resistance of the pulse generator will be found to be approximately 128 ohms.

The foregoing numerical values'are merely representative and should not be construed as limitations of the invention. By applying a kilovolt voltage pulse to a cell having 2-inch long plates spaced at 2 inches, an .exposure time of l() second can be obtained. This approaches the practical operating limit of a Kerr cell having nitrobenzene since the time for such fluid to assume its birefringent characteristics after being subjected to a voltage pulse is approximately lO- second.

GENERAL DESCRIPTION 3 through a light screen assembly, generally designated 4. This assembly includes a low power light source-5 of approximately 25 watts output which projects a thin screen of light, indicated at 6, toward a photocell 7. The screen, which in transverse section may measure .1 inch thick and 4 inches wide, is positioned transversely of path 3 so that the projectile will interrupt the light screen and modulate the intensity of light received by the photocell. This results in a pulse which is amplified and supplied by conductor 8 to a trigger pulse circuit, indicated general ly at 9. This circuit delivers a trigger pulse through conductor 10 to the spark discharge assembly and pulse generator, generally designated 11, resulting in discharge of electrical energy formerly stored in the pulse generator. Projectile 1 is illuminated by light rays 12 emanating from the spark discharge.

Simultaneously, a voltage pulse, which may be as high as 80 kilovolts, is delivered through conductor 13 to the plates 14 of a Kerr cell assembly, generally indicated at 15.

The illuminated projectile can either be directly photographed or photographed by shadowgraph techniques. The latter offer the advantage of recording the shock waves and wake associated with the projectile. The installation of FIGURE 1, is arranged for shadowgraph photography. Thus, the light rays 12 are collimated by condensing lens 16 for illuminating projectile *1. The objective lens 17 is focused on the rear face of the screen assembly at which plane a shadow of the projectile and its attendant shock wave is formed. The resulting shadow picture is focused by the objective lens on the film plane 18 of camera 19.

Light rays, in passing from the objective lens 17 to the film plane 18, pass through the Kerr cell assembly which comprises a polarizer 20 attached to the front of the assembly, the Kerr cell proper with plates 14, and an analyzer 21, attached to the rear of the assembly.

Prior to interruption of the light screen 6 by the projectile, the plates of the Kerr cell are at zero potential and the plane polarized light from the polarizer 20 is almost completely blocked by analyzer 21. Although the Kerr cell transmits a very small amount of light when it is not energized, the light rays of the light screen do not reach the film plane since they are directed at 90 to the axis of the Kerr cell. Care is taken to prevent dust particles from entering the screen assembly and diffusing the light from the light screen.

During most of the time that the high potential pulse is applied to the plates of the Kerr cellsome of the pulse period is required to orient the molecules within the cell-the plane polarized light from polarizer 20 is effectively rotated so that it will pass through analyzer 21 and expose the film at the film plane 18. Since the high voltage pulse is of short duration, the exposure of the film is accomplished in a short time period which maybe in the order of 10- second.

Camera 19 may be a conventional camera with a mechanical shutter which is opened just prior to firing of the projectile launcher and is closed after the film is exposed. In itself, the mechanical shutter does not take an active part in taking the high speed photograph.

DETAILS OF KERR CELL Attention is now invited to FIGURE 2 which shows structural details of the Kerr cell assembly. The Kerr cell proper comprises a tube 22 of heat-resisting, sodaalumina-borosilicate glass within which are positioned the plates 14 to which the voltage pulse from the pulse generator is applied. The plates themselves are made from pure copper and are spaced parallel to each other within a tolerance of about 1.001 inch. The surfaces of the plates facing each other are sandblasted to prevent specular reflection. Since the plates are relatively large, it is desirable to support them at a plurality of points. Thus, each plate is supported by three tungsten leads 23 which are electrically connected to a common junction point 24. The leads are sealed to glass extensions 25 projecting from the sides of cylinder 22. In this way the entire assembly is rendered liquid-tight.

To simplify attachement of the tunsten leads to the copper plates 14, an intervening section of nickel (not shown) may be provided. The machinability of the nickel permits threaded connection to the plates, the nickel sections being welded to the tungsten leads.

The ends of the cylinder are bolted to supports 26 which may be made from phenolic plastic or any other suitable material. At the time of assembly a silicone gasket 27 is interposed between these parts to provide a liquid seal. Optically fiat glass end plates 28 and 29 are attached to the fore and aft supports 26. These are also bolted in place with suitable gaskets.

The polarizer 20 and the analyzer 21 are directly attached to the exterior faces of the end plates 28 and 29, respectively. Both the polarizer and the analyzer are made from positive dichroic sheet material, sold under the trademark Polaroid-type HNZZ, mounted between optically flat sheets of glass. These are not shown in detail since they are conventional and wellknown in the art.

It has been determined that the polarizer and analyzer, when positioned for maximum light transmission, will transmit approximately 7% of an incident ray of light. Interposition of the Kerr-cell between the polarizer and analyzer causes transmission to drop to 6% of the incident light since the fluid within the cell absorbs a small amount of light,

From the standpoint of light transmission it would be desirable, if practical, to utilize Nicol prisms instead of the Polaroid material. However, at the present state of the art, these Nicol prisms would be prohibitively expensive for a Kerr cell of the physical dimensions shown.

The Kerr cell assembly is completed by being filled with highly purified nitrobenzene through filler cap 30a.

SINGLE TRIGGER PULSE CIRCUIT FIGURE 3 shows schematically a circuit for supplying l a trigger pulse to a spark discharge device in association with a transmission line pulse generator and Kerr cell.

The trigger pulse circuit is supplied with power through 110 v. alternating current lines L and L These are connected across the primary 30a of transformer 31. The secondary 32 of the transformer is connected with plates 33 of a double diode rectifier tube 34. Winding 35 of the transformer supplies current to directly heated filament 36 of the diode which serves as a full wave rectifier in conventional manner. Another winding 37 supplies current to heater 33 of thyratron tube 39.

The thyratron tube is a type 2D21 gas filled tube in cluding plate 40, screen grid 41, control grid 42, and cathode 43. The plate 40 is connected through resistor 44 to conductor 45 which may be regarded as the B supply.

Grid 42 is biased relative to cathode 43 through grid resistor 46 and potentiometer 47 to hold the tube below cut-off.

The resistor of the potentiometer 47, in cooperation with the condenser 48, forms the filter for the grid bias supply, whereas condenser 49 serves to filter the B supply. Resistor 50 is a bleeder to improve the power supply voltage regulation.

The thyratron, which is normally non-conducting, may be triggered by manually closing normally open switch 51. This will charge condenser 52 to B potential and will feed a positive voltage transient through blocking condenser 53 to control grid 42. In this way thyratron 39 will be rendered conductive, and the energy stored in condenser 54 will discharge via the primary 55 of pulse transformer 56 and conductors 57-55 The resulting pulse in the secondary 60 of the pulse transformer is applied to trigger electrode 61 which is centrally positioned and insulated from spark gap electrode 62.

If desired, the thyratron may be triggered by a posis a ger 1 F tive pulse applied at 63 to the grid circuit of the thyratron. Triggering is accomplished in this manner when the circuit is used in conjunction with a photocell and amplifier circuit, such as schematically illustrated in FIG- URE 1. Such portions of the circuit, being conventional and well-known, have not been illustrated.

After switch 51 is opened, the charge on condenser 52 equalizes through resistor 64a. The thyratron falls below cut-off by current starvation as condenser 54 discharges sufiiciently to drop the potential of plate below the ionizing potential.

The over-all function of the circuit is to supply a positive trigger pulse of 15,000 volts to trigger electrode 61. This initiates ionization in the gap between spark discharge electrodes 62 and 64, triggering release of energy which was stored in the pulse generator, generally designated 65, by connection of a large negative potential E to charging resistor 66. It is sufficient to understand at the moment that the pulse generator is charged with high potential energy which is released by the spark discharge between electrodes 62 and s4. This establishes a large potential drop across resistor 6'7 which is grounded at 68. A large potential drop is also established between electrode 62 and annular electrode 69, resulting in a second and substantially simultaneous discharge of energy between these latter two electrodes which is visible through the center of the annular electrode 69 and supplies the illumination for the subject being photographed.

PULSE GENERATOR A pulse generator which has been found remarkably effective comprises a single loop of type RG8/U cable having two parallel legs. The free ends of the cable 70 and 71 are positioned adjacent each other with the bight 72 of the cable remote from the ends. The cable, which may be regarded as a series of lumped inductive and capacitive impedances, consists of a central conductor, the ends of which are exposed at 73 and 7d, separated by dielectric insulation from braided shielding, sections of which are shown at 75-77. Shielding sections 75 and 76 are interconnected by conductors 78 and 79 whereas conductor 80 interconnects the adjacent ends of shielding 77.

A metallic film non-inductive type load resistor 81 is interconnected between conductors 79 and 8t) and is also connected across conductors 82 and 83 which are connected to the plates of a Kerr cell, indicated diagrammatically at 84. It will be noted that the shielding is stripped away exposing the insulation at 85 and 86.

It can be established both by calculation and by experiment that a single square wave pulse can be delivered to the plates of the Kerr cell if resistor 31 is made equal to twice the characteristic impedance of the cable. Although the characteristic impedance of RGS/U cable is 52 ohms, when paralleled the characteristic impedance is 26 ohms. Hence, resistor '81 should theoretically be made equal to 52 ohms in order to drive the Kerr cell with a single voltage pulse. Using parameters of such proportions, all other voltage pulses re'flected from the ends of the cable and the resistor will mutually cancel, as will now be explained.

Before application of the trigger pulse to electrode 61, the pulse generator is charged to a high potential by a conventional 50 kilovolt, 2 milliampere power supply through charging resistor 66. The charging potential may lie in the range 35-50 kilovolts depending upon the proportions of the Kerr cell. When fully charged, energy is stored in the pulse generator by stresses in the dielectric between the conductor and shielding of the cable. When the spark discharge is triggered between electrodes 62 and 64-, a voltage pulse of magnitude +E (a sign reversal may be regarded as resulting from the closing of switch 51) travels from the free ends along each leg of the cable to the center region of the transmission line. Here, the voltage pulse encounters a dis continuity of impedance resulting in a reflected wave front of '+'I%E being reflected along each leg 'of the cable toward the free ends. One-third of the original pulses are dissipated in resistor 81. The remaining onethird of the pulses travel along each leg of the cable to bight 72 where they are reflected without change of'sign to form two wave fronts '+%E travelling along each leg of the cable back toward the load resistor 81. By the time these wave fronts reach the load resistor, the other reflected wave fronts do also, having reached the free ends of the cable and been reflected with a reversal of sign so that the reflected wave fronts of identical magnitude meet at the load resistor Where they mutually cancel each other.

It has been seen that, when the impedance of the load resistor is equal to twice the characteristic impedance of the parallel cablegenerator, a single voltage pulse of t-j-E is applied tothe plates of the Kerr cell for a time duration equal to that necessary for the wave fronts to travel from the resistance 81 to the free ends of the cable and back to the resistor. Thus pulse duration is a function of cable length. For a pulse duration of .01 microsecond, it is'recommended that the length of the generator from the free ends to the bight be 7 feet and that the load resistor be positioned in the center of this length.

Advantages obviously can be gained by delivering a square wave pulse larger than B to the Kerr cell. It has been found that a pulse equal to lVsE can be delivered by the pulse generator even though it is only charged to a potential of E if the load resistor is increased from 52 to ohms. This, however, results in wave front reflections in the generator which are not totally cancelled. In fact, using a load resistor of 100 ohms, an uncancelled wave front of /aE will be applied to the Kerr cell sometime after the main pulse of 1 /3 E.

A study of FIGURE 7 will reveal why the uncancelled /313 pulse is not objectionable. This figure illustrates that the percent-transmission of light through the Kerr cell is a mere 3% of maximum Kerr cell transmission when a voltage pulse equal to /3 of the full openvoltage is applied to the cell. It is for this reason that the reflected wave is not objectionable.

The provision of bight 72 is important. Provision of the bight instead of free ends makes it possible to avoid undesirable high voltage corona effects. Further, the transmission line pulse generator is rendered less susceptible to variations due to humidity and other atmospheric effects. The over-all result is a substantial improvement in the over-all consistent operation of the generator.

After the trigger pulse is applied to the electrodes, approximately .01 microsecond elapses before ionization is complete and the are of the spark discharge is fully established. As the arc is established, voltage pulses travel along the legs of the pulse generator until they encounter the impedance discontinuity. It is at this time, approxi mately .005 microsecond after the arc is established, that the square wave pulse is first applied to the Kerr cell. Duration of this pulse will depend upon the proportions of the pulse generator, ashas been-explained. However, for high speed photography a duration of approximately .01 microsecond is desirable. Since the time necessary to orient the molecules of the fluid Within the Kerr cell is relatively small, the time during which emergent light from the Kerr cell passes through the analyzer is equal for practical purposes to the duration of the pulse applied to the Kerr cell.

Illumination of the subject begins as the are is established between the pairs of electrodes and is maintained at peak intensity during the time that light entering the Kerr cell passes through the analyzer. Since the emanation of light from the arc occurs over a time interval of about .2 microsecond, it will be apparent that synchronization of Kerr cell and source of illumination presents no problem and synchronization is fully automatic.

9 MULTIPLE PULSE CIRCUIT FOR DRIVING KERR CELL Shown in FIGURE 4 is a circuit adapted to supply pulses to a Kerr cell 131 at known time intervals. The circuit includes a pulse generator, generally designated 132, which may be the same in all respects as generator 65 of FIGURE 3.

The circuit is charged through conductor 133 to a potential of E. Current flows through resistors 134-136 (which are comparable in function to resistor 66 in FIG- URE 3) to charge condensers 137-139, respectively. One side of con-denser 137 is connected to electrode 140 while the other side is connected by conductor 141 to resistor 142, the opposite end of which is grounded at 143. Simi larly, condenser 138 is connected to electrode 144 and the resistor 142, and condenser 139 is connected to electrode 145 and the resistor 142.

Positioned adjacent each of the electrodes 140, 144 and 145, are electrodes 146148, respectively. These electrodes are grounded through conductor 149.

Associated with the electrodes 146-148 are trigger electrodes 150-152 upon which trigger pulses 153-155 may be impressed seriatim at definite time intervals, usually equal time intervals, by a control circuit which will be described in connection with FIGURE 6. It is sufiicient to understand at the moment that the trigger pulses, each of approximately 15 kilovolts, are impressed at intervals of about 20 microseconds on the various trigger electrodes.

Upon application of trigger pulse 153 to electrode 150, ionization and spark discharge is initiated between electrodes 140 and 146. Formation of an arc between the electrodes discharges the energy of condenser 137 and results in application of a potential diiference of +E across resistor 142 and hence across the pulse generator 132. It will be noted that one end of resistor 142 is connected by conductor 156 to the center conductors of the pulse genera-tor while the shielding of the pulse generator is connected to ground by conductor 157. The nature of a transmission line generator is such that it will deliver a square wave output pulse to the Kerr cell 131 whether it is suddenly charged with energy or energy is suddenly discharged from it. Over-all operation and reflection of wave fronts will be substantially the same.

The charge on condenser 137 slowly decays and gradually relieves the pulse generator of its charge prior to discharge of the second condenser 138 by spark discharge between electrodes 144 and 147 initiated by trigger pulse 154. The second spark discharge again results in application of a voltage pulse of +E to the pulse generator, resulting in delivery of a square wave pulse to the Kerr cell. In similar manner, an arc is established between electrodes 145 and 148 by trigger pulse 155 for discharging the condenser 139 and subjecting the Kerr cell to a third square wave pulse. Obviously, additional pairs of electrodes and condensers could be provided to generate additional square wave pulses for the Kerr cell.

By repeated pulsing of the Kerr cell, it is possible to take a series of pictures of a moving subject at intervals corresponding to the intervals between the trigger pulses 153-155. This is obviously very desirable in connection with ballistics studies.

MODIFIED MULTIPLE PULSE CIRCUIT FOR DRIV- ING KERR CELL The circuit of FIGURE 5 closely resembles the circuit of FIGURE 4 with the addition of a lumped impedance delay line 190, which is connected between conductor 141a and trigger electrode 191. This trigger electrode is associated with electrode 192 and aids in establishing a spark discharge between electrode 192 and electrode 193. It will be noted that electrode 193 and one end of resistor 142a are connected by conductor 194 to conductor 141a. Electrode 192 and the other end of resistor 142a are connected to ground 143:: by conductor 195. Conductor 1560 interconnects the central conductors of the pulse generator 196 with conductor 194. The shielding at the one end of the generator is grounded by conductor 195.

In a manner comparable to FIGURE 4, resistors 134a- 1360 are connected to condensers 137a-139a. A potential of -E is supplied to the circuit through conductor 133a.

Associated with condenser 137a are a pair of electrodes 140m and 146a in addition to a trigger electrode 1510. Upon application of a trigger pulse to electrode 151a, discharge of energy is initiated between electrodes 140a and 146a. This results in release of energy from condenser 137a. and application of potential of +E across resistor 14201. As has been explained in connection with FIG- URE 6, sudden application of potential to the pulse generator causes it to deliver a square wave pulse to the Kerr cell 196a. At the time that the voltage pulse is applied to the pulse generator, it is also applied to one end of delay line 190. As the voltage pulse across the resistor 142a slowly decays, delay line delays and eventually delivers to trigger electrode 191 a trigger pulse which establishes ionization between electrodes 192 and 193. This causes a sudden release of the voltage impressed across the resistor 142a and the pulse generator. The sudden release of energy from the pulse generator, as described with reference to FIGURE 3, causes the pulse generator to generate a square wave pulse which is also applied to the Kerr cell 1961:.

It is important to note that through provision of the delay line and extra set of electrodes 192 and 193, it is possible to make the pulse generator supply two square wave pulses to the Kerr cell for a single trigger pulse applied at 151a. Discharge of energy from condensers 138a and 139:: by their associated electrodes and electrodes 192 and 193 will also result in delivery of pairs of square wave pulses to the Kerr cell. Thus, a circuit to which three trigger pulses are delivered is eifective in delivering six pulses to the Kerr cell.

From the foregoing it will be understood that the pulse generator is effective in delivering a square wave pulse to the Kerr cell whether it is suddenly charged or dis charged. The effect is exactly the same as far as the Kerr cell is concerned except for a change of polarity of the voltage applied to the plates of the cell. Change of polarity, however, is immaterial since the polarizer and analyzer associated with the cell are oriented at 45 positions relative to the field between the plates. Hence, light transmission characteristics of the. installation are not affected by change of polarity.

MULTIPLE TRIGGER PULSE CIRCUIT FIGURE 6 shows a circuit for delivering three 15 kilovolt pulses at spaced time intervals to the trigger electrodes in either FIGURE 4 or FIGURE 5. The pulses are applied to the trigger electrodes through conductors 200-202. The input to the circuit is applied through conductor 203 and may consist of a ISO-volt positive sharp-peak pulse received from the photocell and amplifier circuit, indicating that the projectile has interrupted the light screen and is in position to be photographed.

The input pulse passes through conductor 204, through rectifier 205 and grid resistor 206, to control grid 207 of thyratron tube 208. The thyratron includes plate 209, screen grid 210 and cathode 211.

The thyratron 208 is normally non-conducting at a constant grid bias of 20 volts negative. When the positive pulse is supplied to the grid 207, the negative bias is overcome and the tube becomes conducting. Energy stored in condenser 212 is discharged through thyratron 208 and primary 213 of pulse transformer 214. The sudden surge of current through the primary of the transformer induces a sharp trigger pulse of increased amplitude in secondary 215 of the transformer. It is this pulse which is delivered to conductor 200 and initiates ionization between the first l. 1 pair of electrodes 1'40 and 146 in FIGURE '4'or 140a and 146a in FIGURE 5.

Current'fiow through the'thyratron is terminated by current starvation as the' charge bleeds oif condenser 212. Current starvation is assured by the large size of the plate resistor'216 through which'B 'is supplied to the condenser and the plate of the thyratron.

Simultaneously, the input pulse is also transferred through blocking condenser 217 to conductor 218 and grid 219. This grid controls flow'of current'between plate 220 and cathode 221. These components comprise onehalf of a type 5963 tube. 'The other half'of the tube includes plates 222, grid 223, and cathode224. Plate 220 is-capacitance coupled by condenser'225 to grid 3. Re-

sistors 226, 227 and 228 connect the B+ supplyto plate 220, grid 223 and plate 222, respectively. The cathodes 221 and 224 are connected through a common resistor 229'to grounded conductor 230.

It will be recognized that this tube constitutes a one-shot multivibrator. Prior to application of the input pulseto grid 219, there is a heavy flow of current between plate 222-and cathode 224. Duringsuchtime, grid 223is at substantially the same potential as cathode 224, and plate 220 is at 13+ potential. The input signal to grid 219, however, institutes current fiowbetween plate 220 and cathode 221. This-results in a drop of potential on plate .220 which is transferred as 'a negative gradientthrough condenser 225 to grid 223.

The negative transient applied to grid 223, cuts oifcurrent flow between plate 222 and cathode 224. Thus a change *of state of the multivibratoriseffecte'd and current flow is established between plate 220 and'cathode 221.

The charge on condenser 225 gradually equalizes through resistors 226 and 227, gradually causing'the voltage on grid 223 to rise toward 'B+. which the charge on the condenser is equalized, establishes a definite delay anddetermines when the second trigger pulse will be delivered through conductor 201to the cir- .cuit of FIGURE 4 or FIGURE 5. As flow of current increases between plate 222 and cathode 224, the potential drop across resistor 229 increases, making cathode 221 more positive. This cuts off the current flow in the first half of the tube. Thus the original state of the'multivibrator is re-established.

The resulting operation of the multivibrator produces a square wave voltage pulse in conductor 231. This square wave pulse is differentiated by'condenser 232 and resistor Z33, producing a negative pulse in conductor 234 connect to grid 235 of pentode 236. The pentode includes plate 237 and cathode 238 in addition to screen grid 239 which is connected through resistor 240 to 13+ and condenser 241 to ground.

The pentode 236 serves to amplify and invert the negative pulse from the differentiating circuit and causes a proportionate amplifiedcurrent to flow through primary 24101 of coupling transformer "242. The secondary 243 'of the transformer is connected across resistor 244 which 'acts as a load resistor. As a result, the pulse from the differentiating circuit is amplified and delivered to conductor 245 and through resistor 246 to grid 247 of thyratron 248. The positive wave pulse applied to the grid of the thyratron rendersit conducting and, with the aid ofpulse transformer 249, produces a trigger pulse of kilovolts in conductor 201 in a manner similar to that described with reference to thyratron 208. For this reason, the .details of this portion of the circuit will not be described again.

Simultaneously with the delivery of the input pulse to conductors 204 and 218 it'is also delivered to conductor 250. This conductor is connected to grid 251 of a second multivibrator, generally designated 252. This multivibrator is in turn connected to pentode 253 which in turn is transformer coupled to the thyratron 254. Operation of multivibrator 252, the associated pentode 253, and thyratron 254, is substantially similar to that described The time, during on grid 251.

above, and for'this reason will not be described in detail again. The resulting operation of the circuit is the delivery of a 15 .kilovolt trigger pulse to conductor 202.

Certain characteristics of the overall circuit can "now be considered. .First of all, it will be noted that the initial bias on grid 219 is established by potentiometer 255 across which a shunt resistor 256 is connected. The potentiometer and shunt resistor are'connected in parallel through resistor 257 to the B+ supply and are connected by conductor 258 to ground. In this way, the .bias on grid can be adjusted.

nificantly different duration supplied to associated pentodes 239 and 253, respectively. Thus the individual bias control for the multivibrators in addition to the adjustable condensers of significantly different size, cause different delays in the delivery of the trigger pulses to associated conductors, 201 and 202, respectively.

A constant bias of 20volts-negative is supplied to conductors 261-264 and is impressed on the grids of the thyratrons208, 248 and 254 by resistors 265, 244 and 246, and 266 and267, respectively. The same bias voltage is supplied to thegri-ds of tub-es 236 and 253 through resistor-s 233 and 268, respectively.

To illustrate the spacing of the trigger pulses, the input pulse at 203 causes a trigger pulse to be delivered almost-instantaneously to conductor 200; about 20 microseconds'later, a second trigger pulse to be delivered to conductor 201. Approximately 20 microseconds thereafter, a third triggerpulse is delivered to conductor 202. These time intervals are exemplary only and may be varied tosuit the particular series of photographs to be taken.

In connection withthe discussion of FIGURES 4 and 5, no mention was made of a source of illumination for the subject being photographed. To permit multiple exposures, conventional Xenon electronic .flash tubes may be energized at the necessary time intervals by means of a circuit which is substantially identical to that .of FIGURE 6. 'For such use, the trigger pulse would be used, not to pulse the Kerr-cell, but to-energize the light source in synchronization with the pulses delivered to the vKerr cell by circuits, such as FIGURES 4 and 5.

It is deemed desirable to provide intervals of illuminationfor making multiple exposures rather than using continuous illumination because the Kerr cell, when not pulsed, is nevertheless capable of transmitting enough plane polarized light to the analyzer to fog high speed film at the film plane.

PARAMETERS The following parameters, although not limitations of the invention, have beenused in the foregoing'circuits:

FIGURE 3 Transformer 31 Power transformer 320 0 320 volts D.C. Rectifier tube 34 Type 5Y3. Thyratron tube 39 Type 2D2l. Resistor 44 330,000 ohms, 1 watt. Resistor 46 100,000 ohms, V2 watt.

Potentiometer 47 20,000 ohms, 2 watts. Condenser 48 50 aid, 50 volts D.C. Condenser 49 20 id, 600 volts D.C. Resistor 50 220,000 ohms, 2 watts. Condenser 52 .1 fd, 600 volts D.C. Condenser 53 .01 ,ufd., 600 volts D.C. Condenser 54 2 ,ufd., 600 volts D.C.

Transformer 56 Winding ratio 35/ 1 step-up; 400 V. input; secondary insulated for 20,000 volts.

Resistor 64 22 megohms, /2 Watt.

Resistor 66 100 megohms, 25 watts.

Resistor 67 100,000 ohms, 2 watts.

Resistor 81 100 ohms, 2 watt, metallic film.

FIGURE 4 Resistor 134 10 megohms, 50 kilovolts.

Resistor 135 10 megohms, 50 kilovolts.

Resistor 136 1O megohms, 50 kilovolts.

Condenser 137 1 5000 ,u/Lfd., 50 kilovolts.

Condenser 138 1 5000 id, 50 kilovolts.

Condenser 139 1 5000 ,UqlLfd., 50 kilovolts.

Resistor 142 26 ohms, 50 kilovolts.

Potential -E 50 kv. from 50 kv. 2 milliampere power supply.

1 Capacitance should be equal to or greater than 10 times the capacitance of pulse generator.

2 For 7 ft. generator of RGS/U Cable.

FIGURE Delay line 190 Characteristic i m p e d a n c e 820 ohms; travel time .4 asee; capacitance 500 aid; inductance 338 Condenser 137a 1 Condenser 138a 1 5000 aid 50 kilovolts. 5000 pufi, 50 kilovolts.

Condenser 139a 1 5000 i id, 50 kilovolts. Resistor 142a 26 ohms, 50 kilovolts. Potential -E 50 kilovolts from 50 kv. 2 milliampere power supply.

Capacitance should be equal to or greater than times the capacitance of pulse generator.

2 For 7 ft. generator of RGB/U Cable.

FIGURE 6 Thyratron 208 Type 2D21. Condenser 212 .25 id, 600 volts D.C.

Transformer 214 Winding ratio 35/1 step-up; 400 v. input; secondary insulated for 20,000 volts.

Resistor 216 500,000 ohms, lwatt.

Condenser 217 100 ,a fd, 400 volts.

Condenser 225 500 id, 400 volts D.C.

Resistor 226 7,0001ohms,2watts.

Resistor 227 2 megohms, /2 watt.

Resistor 228 7,000 ohms, 2 watts.

Resistor 229 3,000 ohms, 2 watts.

Condenser 232 .0001 ufd, 400 volts.

Resistor 233 47,000 ohms, /2 watt.

Pentode 236 Type 6AN5.

Resistor 240 22,000 ohms, 1 watt.

Condenser 241 .02 ,ufd., 400 volts.

Transformer 242 1/1 ratio; 400 volts between windings; inductance per winding 4 millihenries.

Resistor 244 1 megohm, /2 watt.

Resistor 246 10,000 ohms, /2 watt.

Thyratron 248 Type 2D21.

Transformer 249 Winding ratio 35/ 1 step-up; 400 v. input secondary insulated for 20,000 volts.

Multivibrator 252 Type 5963.

Pentode 253 Type 6AN5. Thyratron 254 Type 2D21. Potentiometer 255 100,000 ohms, 2 watts. Resistor 256 100,000 ohms, /2 watt. Resistor 257 100,000 ohms, /2 watt. Resistor 265 10,000 ohms, /2 watt. Resistor 266 1 megohm, /2 watt. Resistor 267 10,000 ohms, /2 Watt. Resistor 268 47,000 ohms, /2 watt.

CONCLUSION From the foregoing description, it will be evident that means have been provided for taking pictures of high speed projectiles in flight. Time exposures can be varied to suit the particular subject and can be made as short as .01 microsecond, or less.

A shadowgraph of a high speed projectile is shown in FIGURE 8. The projectile 1 is preceded by a luminous wave front 300 and is followed by a turbulent wake 301. FIGURE 8 was taken with an exposure time of .09 microsecond and is a shadowgraph of a .6 inch diameter steel sphere travelling at a velocity of 5,300 feet per second.

From the foregoing it will be understood that improvements have been made not only in the over-all installation but in the components of the system such as the pulse generator, circuits for pulsing the Kerr cell, trigger pulse circuits, and spark discharge devices. The entire invention represents a significant advance in the art of high speed photography and one which serves a vital purpose in the development of missiles for the defense of our country.

The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will occur to those versed in the art as likewise will many variations and modifications of the preferred embodiment of the invention, all of which may be achieved without departing from the spirit and scope of the invention.

Having described my invention, I claim:

1. A transmission line pulse generator comprising a length of cable having a central conductor surrounded by insulation and shielding over the insulation, an interruption in said shielding separating said shielding into two sections, a load impedance connected in series With said shielding and bridging said interruption, means to create an electrically charged condition between said central conductor and said shielding, and means to abruptly connect said central conductor and said shielding together to generate a voltage pulse.

2. A pulse generator as defined by claim 1 in which said length of cable defines two parallel legs with the shielding interrupted at the mid-point of the legs.

3. A pulse generator as defined by claim 2 and, in addition, conductors interconnecting the corresponding ends of the shielding on either side of the interruptions at said midpoints, a grounded conductor interconnecting the shielding at the ends of said length of cable, and said load impedance being interconnected between said first mentioned conductors.

MILTON O. HIRSHFIELD, Primary Examiner.

HERMAN K. SAALBACH, Examiner. 

1. A TRANSMISSION LINE PULSE GENERATOR COMPRISING A LENGTH OF CABLE HAVING A CENTRAL CONDUCTOR SURROUNDED BY INSULATION AND SHIELDING OVER THE INSULATION, AND INTERRUPTION IN SAID SHIELDING SEPARTING SAID SHIELDING INTO TWO SECTIONS, A LOAD IMPEDANCE CONNECTED IN SERIES WITH SAID SHIELDING AND BRIDGING SAID INTERRUPTION, MEANS TO CREATE AN ELECTRICALLY CHARGE CONDITION BETWEEN SAID CENTRAL CONDUCTOR AND SAID SHIELDING, AND MEANS TO ABRUPTLY CONNECT SAID CENTRAL CONDUCTOR AND SAID SHIELDING TOGETHER TO GENERATE A VOLTAGE PULSE. 